Heat resistance of Cr?Ni and Cr?Ni?Mo steels with boron additions

Abstract

1. At temperatures below the equcohesive temperature, i.e., with transcrystalline fracture, the long-term strength of Cr−Ni and Cr−Ni−Mo steels increases continuously with increasing boron additions up to 0.005%. At temperatures above the equicohesive temperature, where fracture is intergranular, the long-term strength of the steels is highest at boron concentrations of 0.001–0.0018%. 2. The following mechanisms of the increase in the long-term strength with small boron additions were established: a) small boron additions increase the strength of the grain boundaries, which is manifest at temperatures above the equicohesive temperature; b) small boron additions slow down the diffusion processes of precipitation of excess phases in the boundaries and within the grains during creep; c) additional strengthening of the grain boundaries evidently occurs at concentrations of 0.004–0.005% B due to precipitation of dispersed borides.